Maintenance of cell volume is a homeostatic process expressed in all animal cells. The necessary water movement across the plasma membrane is coupled to solute transport mechanisms that are also involved in other cellular activities such as the accumulation of glucose, pHi regulation or vectorial fluid transport. The corneal endothelium is endowed with an active ion pump mechanism responsible for vectorial fluid transport and hence the maintenance of corneal hydration, a determinant of corneal transparency. Since this fluid transport and cell volume regulation must be intertwined, it is imperative to establish the mechanisms of volume maintenance and regulation to obtain a complete understanding of the fluid transport. In addition, it is of clinical significance to understand how fluid transport is affected during times of osmotic stress such as during contact lens wear and diabetes. The focus of this study is to establish the principles of volume regulation upon acute and sustained disturbances leading to short and long term volume regulations, respectively/ Short term volume regulation will be studied by following changes in cell volume and intracellular ionic activities to identify the mechanisms and second messengers involved. Experiments will employ anisosmotic perturbations, acidosis, hypoxia, ion substitutions and transport-specific inhibitors. Long term volume regulation will be examined in the context of adaptation to sustained anisosmotic loads by adjusting the levels of organic osmolytes (such as amino acids, myo-inositol and sorbitol). Isolated in vitro rabbit corneas and cultured bovine endothelial cells that are confluent and polarized on either glass coverslips or permeable supports will be used. The dynamics of cell volume will be measured using athe sensitive light scattering technique and by following the concentration of a fluorescent dye used as a volume marker. The long term goal of this study is to understand the interaction of the fluid transport, pHi regulation and volume regulation activities of the corneal endothelium under normal conditions and during metabolic stress, hyperglycemia and anisosmotic loads.